The present invention generally relates to radio local loop communication systems and, more particularly, to cell splitting in such systems.
The radiocommunication industry has made phenomenal strides in commercial operations in the more heavily industrialized countries as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry's growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
Cellular radiocommunication systems have provided a large portion of this growth due to, for example, their ability to support mobile communications. However, cellular systems comprise only a portion of the radiocommunication universe. Another type of radiocommunication system is commonly known as a radio local loop (RLL) or wireless local loop (WLL) system. In RLL systems, shared radio connections replace, at least partially, the more conventional wire connections between remote units (e.g., telephone sets) and the local exchange. To implement these radio connections, subscriber radio terminals (SRTs) support a subscriber end of the radio link (e.g., connected to a standard telephone set) and radio base stations (RBSs) are connected to the local exchange. The air interface between RBSs and SRTs can be implemented using any known cellular technology, for example, NMT, GSM, TACS, D-AMPS, etc. By replacing wire connections with radio connections, system installation can be performed quickly and cheaply, particularly at rural sites where distances between subscribers and local exchanges make running copper wire prohibitively expensive or high density areas where wiring complexity increases costs.
Like many popular innovations, radiocommunication systems could be described as victims of their own success in the sense that demand for wireless services threatens to exceed the capacity of installed systems' in certain areas. One solution used in cellular systems to expand capacity is known as cell splitting. By reusing the same frequency or group of frequencies in only cells which are a certain minimum distance apart, the interference contributed to signals within those cells from the so-called co-channel cells is reduced to a level which provides acceptable receive signal quality. Thus, in many cellular systems frequency reuse patterns have been adopted to provide geographical restraints on the reuse of frequencies to limit co-channel interference. However, frequency reuse patterns also limit capacity in radiocommunication systems which are allocated only a finite bandwidth within the radio frequency spectrum. Thus, cell splitting, i.e., dividing a cell into several smaller cells that use different groups of frequencies for communication, increases overall system capacity by increasing frequency reuse. Note that this allows the co-channel interference ratio to remain the same.
In cellular systems, each cell is associated with at least one control channel which supports various overhead functions including paging and access to the system. In cellular systems, the selection of a particular base station as a serving base station for a particular mobile station can be made based upon, for example, the strength at which the mobile station receives each base station's control channel. When a cell split occurs in a cellular system, each newly created cell will have its own control channel associated therewith. In this way, mobile stations can quickly adapt to the change in system structure by listening to the new control channels as well as those which existed before the cell split. Accordingly, a cellular system's adaptation to cell splitting is relatively uncomplicated, at least from the mobile station's point of view.
Like cellular systems, many RLL systems require post-installation expansion to handle additional subscribers. Unfortunately the cell splitting technique used in cellular systems is not readily adaptable to RLL systems. One reason is that cell splitting was designed for systems which support subscriber mobility, but RLL systems are designed to support subscriber units which have limited mobility due to, for example, regulatory reasons and physical system limitations of local exchanges. For example, while cellular systems provide subscriber registers on the trunk side of the exchange which allows calls to be routed to other exchanges as necessary, existing RLL systems use subscriber line registers which are disposed in the switch interface module (SIM), which interfaces the RBSs with the local exchange. Since the SIM is disposed on the subscriber's side of the local exchange, access to the subscriber line registers is localized and routing calls to other local exchanges or SIMs is not possible. To prevent SRTs from attempting to access restricted RBSs, each SRT is manually programmed with a code identifying the cell or cells which it may access. When cell splitting occurs in RLL systems, new cells are created and thus SRTs may be restricted to a new RBS or group of RBSs. This requires manually reprogramming each affected SRT with a new code. In addition to cellsplitting, other network reconfigurations may also occur which necessitate reprogramming of SRTs. However, manual reprogramming of SRTs may take, for example, several hours per subscriber thus making changes in the RLL network virtually impossible for practical and economical reasons.
Another drawback to conventional RLL systems is that since each SRT is restricted to, for example, a single cell with which it can communicate, there are frequently periods during which an SRT will be unable to provide a connection. For example, an SRT may lose contact with its cell due to certain weather conditions or if an object is moved into a position which completely blocks the SRT from radio communication with its assigned RBS. Another example would be if the RBS is temporarily out of service for maintenance or system reconfiguration purposes.